Ground Fault Detection transformer sizing
Ground Fault Detection transformer sizing
(OP)
I have a substation with a transformer having a 34.5KV ungrounded tertiary winding which supplies a wye-connected reactor and a station aux transformer. I am planning to design a ground fault detection system using a grounded-wye to broken delta transformer. Everything I have read indicates that the ground detection transformer and the resistor which is placed across the broken delta should be sized based upon the system charging current in order in prevent ferroresonance and other issues. However, the primary means of determining the system charging current appears to be either measuring it or using previous experience. The little real-world data I have seen published generally goes no higher than 13.8KV and is mainly derived from medium-voltage distribution systems. I think it is reasonable to expect that the system charging current in the system I am describing will be very low, but I'd like to have some sort of estimate based in reality. :) Does anyone have any useful information or a good reference?






RE: Ground Fault Detection transformer sizing
For a 5 Amp ground fault threshold I would guess 167 KVA or 250 KVA, depending on how fast you will clear a fault.
You won't have much charging current for station service and a reactor.
Bill
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"Why not the best?"
Jimmy Carter
RE: Ground Fault Detection transformer sizing
If you want to keep it ungrounded, you must use Potential Transformer
with secondary open delta with resistor , to install 59N to detect fault-to-ground.
See Bassler guide to calculate resistor.
RE: Ground Fault Detection transformer sizing
As for ungrounded with ground fault detection vs high impedance grounded, is there a practical difference in the system described? I have read the Basler guide, and they make certain assumptions without explaining why. For example, the transformer size.
RE: Ground Fault Detection transformer sizing
Why do you worry about 5A? Your objective is only to detect fault-to-ground.
RE: Ground Fault Detection transformer sizing
RE: Ground Fault Detection transformer sizing
1- Are there loads beside reactor and station aux transformer?
2- your SAT is 34.5/ low voltage and delta HV/ Yn low voltage?
3- 34.5kV system is a generating busbar?
4- How is the unifilar?
I have seen ungrounded system (0 A),grounded by high-impedance (5-30A), grounded by middle impedance(100-600A), grounded by low impedance (1F < = 3F).
RE: Ground Fault Detection transformer sizing
Your intention is to install a ground fault detection system on a DELTA connected network using WYE-BROKENDELTA transformer.The estimation of charging current is needed if your system is a high resistance grounded system so that your NGR is designed to allow little more than 3xcharging current of the network.But here your network is DELTA ungrounded.Therefore,what you need is three VTs each having maximum 500VA (very typical)connected primary in WYEGND and the secondary in BROKENDELTA across a simple voltage relay(ANSI function 59N).BASLER model BE1-59N is very widely used low cost relay.The resister connected accross this voltage relay is nothing to do with the charging current because it is the stabilizing resister to mitigate any possible ferroresonance.The reason for saying that is whether you connect or not this resister,the relay will sense the zero sequence voltage during a ground fault and it will alarm or trip depending on its settings.Therefore,first calculate the maximum possible voltage during a ground fault accross the stbilizing resister considering the turns ratio of the VTs and then size the resister for the full thermal rating of the VT.Donot oversize the resister because it may "invite" ferroresonance.